1. Statement of the Technical Field
The inventive arrangements relate to coherent demodulators, and more particularly to coherent demodulators that use adaptive channel trackers
2. Description of the Related Art
In digital data communication systems, transmit symbols must be reconstructed from a received sequence of transmitted symbols. A common difficulty which must be overcome in such systems is the problem of inter-symbol interference (ISI), as is frequently caused by multi-path propagation. It is well known that ISI can be reduced by lowering the symbol transmission rate. However, this leads to lower efficiency and can be avoided by using an equalizer or a maximum likelihood Viterbi algorithm which effectively compensate for the ISI problem. The equalizer effectively inverts the effects of the channel by functioning as a system in series with the channel.
In order to function effectively, an equalizer must have some knowledge of the channel. However, real mobile radio channels are constantly changing and therefore the equalizer must be constantly updated with new information about the current state of the channel. This function is performed by a channel tracker (sometimes referred to as a channel estimator) which implements a channel tracking algorithm. The combination of the equalizer and the channel tracker is sometimes referred to as an adaptive equalizer.
The optimum bandwidth to be used for a filter which is matched to the modulation scheme will vary depending on the Doppler shift associated with a received sequence of transmitted symbols. Doppler shift is the frequency shift experienced by a radio signal when a wireless receiver and/or transmitter is in motion. Doppler shift can result in Doppler spread in the frequency domain. Accordingly, the adaptation time of processes which are used by channel trackers are preferably faster than the rate of change of the channel. Current methods used for adaptive channel tracking are processing intensive and include Kalman filters, pilot sequences and/or multiple filter banks. Accordingly, it would be desirable to provide adaptive channel tracking that is quick, simple and effective.